Why do Apple computers have so few spyware and virus problems?

I have been using both PCs and Macs for many years now, and have always made sure to run both spyware and virus protection programs on the PCs I use running XP.
On my macs, I have never thought about this and I have never had any kind of problems related to either of these.
I’m not trying to start a debate here, but this just seems to be how it is. Why are Apple computers not plagued by this? Is Vista helping the problem?

because unix is the basis of the mac os. it has been around over 50 years and had the bugs mostly worked out of it. Where windows has been around a little over 20 years and they keep changing the source code and starting over every few years to keep up with customer demands for new gadgets. The core is based on windows nt technology but the source code is new.

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